Work of our members

Sundial with uniform hour line spacing

The figure below shows a horizontal sundial for a latitude of 52 degrees.
The circular boundary has its centre in the red dot on the XII hour line.
This centre may be chosen arbitrarily and does not even have to be on XII hour line.
The angles between the hour lines are not of course equal, but we will change that.

From the centre of the circle, draw lines every 15k degrees. Here, 15k equals 20 degrees, and k is therefore 1.3333.
These lines will be our new hour lines, and we draw markers where these lines intersect the original hour lines.

Draw a flowing line through the markers and erase the original hour lines.
The figure below shows what remains.

The part of the sundial coloured grey is fixed in the north-south direction.
The style is where the original hour lines intersect.
A second, rotating plate with the new hour lines is placed under the cardioid plate.
The sundial is read where the pole style shadow intersects the edge of the cardioid.
The rotating plate with hour lines, which are uniformly spaced, enables us to adjust for longitude and for the equation of time.

An example of such a sundial, with construction plans, is shown below. Our member Thijs de Vries built this sundial 1988. He chose a k of 1.4, resulting in hour lines spaced 21 degrees.

The American Fred Sawyer developed the original idea for this type of sundial in the late seventies. It is known as the “equant sundial”.